Iterative polygon refinement.
Inspired by iterative bounding box refinement in [1]}, we refine the vertices in each polygon in the decoder layer-by-layer.
We use a prediction head (MLP) to predict relative offsets \((\Delta x, \Delta y )\)  from the decoder embeddings and update the polygon queries for the next layer.
Both decoder embeddings and polygon queries input to the first layer are randomly initialized.
We visualize this iterative refinement process in Fig. REF .
The final predicted labels are used to select valid queries and visualize their position after each layer.
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